The disbelief comes mainly from the assumption that there is an exclusive, that is, one-on-one cause and effect relationship between teacher quality and learning outcomes. If someone believes that the only factor that affects how students perform in exams is the teacher then the above ratings of teachers in New York compared against how the students perform truly become incomprehensible. This is unfortunate since the news that most teachers in New York are seen as effective should be actually good. It ought to be welcomed with a sigh of relief and not disbelief. Sadly, there are loud voices out there that tend to blame the lackluster performance of students in standardized exams to ineffective teachers. And to these voices, the results of the recent evaluation only mean one thing - the evaluation is wrong.
There are other factors (oftentimes, these are much more important than teacher quality) that influence learning. One simply has to glance, for example, at an article written by Willingham in 2012 o realize this truth:
Willingham's answer to the above question is shown below:
The New York evaluation of teachers only addresses the teacher's side. The evaluation does not say that ninety percent of teachers in New York can magically make the effects of poverty on education disappear....